Zendesk said Thursday a hacker gained access to support information for some customers of its online help desk software.

"We've become aware that a hacker accessed our system this week," wrote Mikkel Svane, Zendesk's CEO. "As soon as we learned of the attack, we patched the vulnerability and closed the access that the hacker had."

The company has more than 20,000 customers, including large enterprises such as Sears, Xerox, and Groupon. Svane wrote that the three affected customers have been notified, but he did not identify the businesses.

However, Twitter wrote that it was "emailing a small percentage of Twitter users who may have been affected by Zendesk's breach. No passwords involved."

Pinterest and Tumblr was also affected. A Pinterest spokesman shared an advisory sent to its users that said in part, "Unfortunately your name, email address, and subject line of your message were improperly accessed" in Zendesk's breach. Pinterest advised users to be wary of suspicious emails asking for their password.

Tumblr told its users in an advisory that while much of the exposed information is "innocuous," it could allow hackers to associate a person's email address with the address of their blog. "We're working with law enforcement and Zendesk to better understand this attack," Tumblr's notice said.

Zendesk's investigation so far shows that the hacker "downloaded email addresses of users who contacted those three customers for support, as well as support email subject lines," Svane wrote.

Email addresses are useful for hackers, as it gives them a direct way to deliver malicious links and content that could attack a victim's computer if it is running out-of-date software.

A range of attacks in the last few weeks have targeted software vulnerabilities in Oracle's Java and products from Adobe Systems. Although those companies have issued patches, many computers may not have been updated yet.

Zendesk is just one of a number of companies over the last few months to disclose network breaches. It comes just after a lengthy report on cyberespionage released earlier this week by security vendor Mandiant alleging a Chinese military unit waged an extensive seven-year hacking campaign against 141 companies and organizations.

Svane wrote Zendesk is committed to working with law enforcement on the breach and to "bring anyone involved to justice."

"We're incredibly disappointed that this happened and are committed to doing everything we can to make certain it never happens again," he wrote. "We've already taken steps to improve our procedures and will continue to build even more robust security systems."